Typical hydrocarbon shale formations are significantly different from conventional reservoirs, inasmuch as they are characterized by very low permeabilities, for example, with the permeability values in the nano-Darcy range (Cipolla 2009). To extract hydrocarbons from these formations, horizontal wells are often stimulated by multi-stage fracturing (Liu, Liu et al. 2015, Yushi, Shicheng et al. 2016)). Conventional hydraulic fracturing in horizontal wells is undertaken by placing several transverse fractures within a single stage (Holditch 2006), in a process that involves an interaction between induced and natural fractures (Dahi-Taleghani and Olson 2011). It is generally understood that the success of a fractured shale horizontal well is a function of the nature of the conductive fracture network, as determined by a parameter known as a stimulated reservoir volume (SRV) (Mayerhofer, Lolon et al. 2010, De Barros, Daniel et al. 2016). The induced fracture network is made up of reopened natural fracture (NF) networks and induced hydraulic fractures (HFs) formed by the opening or slippage of fractures initiated by the release of stresses resulting from hydraulic fracturing treatments (Gale, Reed et al. 2007, Cho, Ozkan et al. 2013). In this context, NFs can be understood as potential weak points for the initiation of HFs that extend the fracture network (Laubach 2003, Clarkson 2013, Kresse, Weng et al. 2013).
It has been widely reported that the existence of NFs in reservoir rock may change the direction or nature of induced HF propagation (Daneshy 1974; Anderson 1981; Zhou, Chen et al. 2008; Guo, Zhang et al. 2014). Similarly, a wide variety of theoretical approaches have been applied in an effort to characterize the nature of NF and HF interactions (Lam and Cleary 1984; Akulich and Zvyagin 2008; Shakib 2013; and, Chuprakov, Melchaeva et al. 2014). Much of this analysis fails to take into account the induced stress caused by multiple fractures, although efforts have been made to do so (East, Soliman et al. 2011; Cheng 2012; Zeng and Guo 2016)
The nature of a selected completion pattern is understood to have an important effect on the formation of complex fracture networks (East, Soliman et al. 2011, Manchanda and Sharma 2014, Wu and Olson 2015, Wang, Liu et al. 2016, Zeng and Guo 2016). One approach to completions in shale formations involves simultaneous fracturing of multiple perforation clusters in a horizontal wellbore, generally undertaken with essentially the same perforation parameters at perforation clusters that are relatively closely spaced, so that all of the perforation clusters initiate and propagate HFs simultaneously. In this way, the induced stresses of HFs may encourage the creation of stress interference between the successive fractures, thereby promoting fracture complexity (East, Soliman et al. 2011, Wu and Olson 2015). A different approach is known as alternate fracturing, in which a third fracture is placed between the two previously propped fractures. Altemate fracturing is thought to promote the introduction of complex fracture networks (Roussel and Sharma 2011, Manchanda and Sharma 2014). A wide variety of alternative fracturing techniques have been disclosed, many of which employ specialized tools (East, Soliman et al. 2011; Zeng and Guo 2016).
In the context of the present disclosure, various terms are used in accordance with what is understood to be the ordinary meaning of those terms. For example, a “reservoir” is a subsurface formation containing one or more natural accumulations of moveable petroleum or hydrocarbons, which are generally confined by relatively impermeable rock. In this context, “petroleum” or “hydrocarbon” is used interchangeably to refer to naturally occurring mixtures consisting predominantly of hydrocarbons in the gaseous, liquid or solid phase. A “zone” in a reservoir is an arbitrarily defined volume of the reservoir, typically characterised by some distinctive properties. Zones may exist in a reservoir within or across strata or facies, and may extend into adjoining strata or facies. “Fluids”, such as petroleum fluids, include both liquids and gases. Natural gas is the portion of petroleum that exists either in the gaseous phase or in solution in crude oil in natural underground reservoirs, and which is gaseous at atmospheric conditions of pressure and temperature. Natural gas may include amounts of non-hydrocarbons. A “chamber” within a reservoir or formation is a region that is in fluid/pressure communication with a particular well or wells.
In reservoir rock, natural and/or induced fractures may form an interconnected network of fractures referred to as a “fracture network.” A fracture network is “complex” when it comprises a significant number of interconnected fractures extending in alternative directions, or along alternative planes. As used herein, the phrase “fracturing interval” refers to a portion of a subterranean formation into which a fracture or fracture network may be introduced. In the context of hydrocarbon reservoirs, particularly gas reservoirs, “shale” is a fine-grained sedimentary rock that forms from the compaction of silt and clay-size mineral particles that is commonly called “mud”. This composition places shale in a category of sedimentary rocks known as “mudstones”. Shale is distinguished from other mudstones because it is fissile and laminated. “Laminated” means that the rock is made up of many thin layers. “Fissile” means that the rock readily splits into thin pieces along the laminations.